A new species of Allostoma (Platyhelminthes, Prolecithophora) from the coast of Japan

The genus Allostoma von Beneden, 1861 (Pseudostomidae Graff, 1904) consists of generally small, often conspicuously colored species. These organisms are typically found on hard bottoms, in gravel, or among algae in marine and brackish-water environments. The Pseudostomid taxon encompasses approximately 55 known species globally, spanning regions across Europe, Asia, and North and South America. In Japan, however, only three species of this taxon have been reported to date: Allostoma durum (Fuhrmann, 1896), Allostoma densissimabursa (Omi, 2020) comb. nov., and Cylindrostoma monotrochum (Graff, 1882). Samples of seaweed and sand were collected from the coast of Shimane Prefecture in western Japan. Turbellarians were subsequently isolated and examined both in their live state and in histological sections. This investigation led to the description of a new species, Allostoma matsueensis sp. nov., characterized by a pear-shaped granular vesicle, non-sclerotized retractile penis, a narrow rigidly sclerotized conical spermatic duct, a supraterminal female pore, and a thin membraned bursa. The species further exhibits a yellow-to-pale brown body. This discovery constitutes the first record of the genus Allostoma from the western marine coast of Japan.